Composite storage tank having double wall characteristics

ABSTRACT

A composite storage tank is built which has double wall characteristics. A method of building the tank comprises the steps of applying a viscous resinous core material to a support surface and then winding onto, into or along with the viscous resinous core material discrete filament bundles. The resinous core material encapsulates the filament bundles, but does not significantly penetrate into an interior of the bundles. The substantially resin-free interiors provide flow paths for the ready detection of wall leakage. The resultant composite storage tank has double wall characteristics. A crack or hole in either an inside or outside surface which extends at least to the encapsulated filament bundle interiors will alert the tank&#39;s operator/owner to a leakage problem.

This invention relates to a composite storage tank. More particularly,the invention relates to a composite storage tank having double wallcharacteristics and a method of building the tank.

BACKGROUND OF THE INVENTION

A need became apparent in the 1980's for a reliable underground bulkstorage tank system. The then industry standard storage tanks weresingle wall. A major leak or tank wall collapse could result in 20,000or more gallons of liquid, e.g. gasoline, being spilled. Even a minorleak could cause substantial environmental damage over time.

When the leakage problem inherent with the single wall storage tanksystems became apparent, the tank industry began building double wallstorage tank systems. An inner wall contains the stored liquid while anouter wall encompassing the single wall provides back-up or secondarycontainment capability. A space between the walls is monitored for wallleakage. Examples of such double wall tank systems are found in U.S.Pat. Nos. 4,653,312, 4,739,659, 4,875,361, 4,925,046, 5,017,044,5,020,358 and 5,167,142. Such systems have proved reliable. Many of thedouble wall storage tank systems now being sold are based on theirdesigns.

Now that the reliability of double wall tank systems have been generallyrecognized, the industry has begun to focus on producing the tanks moreeconomically. A savings in raw material costs and a savings in laborcosts are possible.

In accord with a need for a more economical storage tank system, therehas now been developed a method of building a composite storage tank.The tank is built by a method which results in a savings in both rawmaterial and labor costs. The resultant storage tank has double wallcharacteristics which gives it performance attributes that make it aviable alternative to current double wall storage tank systems.

SUMMARY OF THE INVENTION

A method of building a composite storage tank comprises the steps ofapplying a viscous resinous core material to a support surface andwinding onto, into or along with the viscous resinous core materialdiscrete filament bundles. The resinous core material encapsulates thefilament bundles, but does not significantly penetrate into an interiorof the bundles. The substantially resin-free interiors provide flowpaths for the ready detection of wall leakage. The resultant compositestorage tank has double wall characteristics. A crack or hole in eitheran inside or outside surface which extends at least to the encapsulatedfilament bundle interiors will alert the tank's operator/owner to theproblem.

BRIEF DESCRIPTION OF THE INVENTION

FIG. 1 is an environmental view of a half-section of composite storagetank of the invention in the process of being built on a male mandrel.

FIG. 2 is a view in section showing in detail a portion of the compositestorage tank wall while still on the mandrel of FIG. 1 in an earlyformation stage.

FIG. 3 is a view in section of a portion of the composite storage tankwall while still on the mandrel of FIG. 1.

FIG. 4 is a partial view in section of another composite storage tankwall of the invention wherein a gel coat forms an underside of the wall.

FIG. 5 is a partial view in section showing a preferred compositestorage tank wall having an inner and outer fibrous reinforced resinousmaterial.

FIG. 6 is a partial view in section showing in detail another compositestorage tank wall wherein two plys of filament bundles at slight crossangles are used.

FIG. 7 is a partial view in section showing still another compositestorage tank wall wherein two plys of filament bundles at substantialright angles to one another are used.

FIG. 8 is a sectional view of the composite storage tank wall of FIG. 7taken along line 8--8 thereof.

FIG. 9 is a partial view in section showing in detail a compositestorage tank wall wherein a foamed core is provided.

FIG. 10 is a side view of a composite storage tank of the invention withsupport ribs added for enhanced strength.

FIG. 11 is a partial view in section of the composite storage tank ofFIG. 10 taken along line 11.

DETAILED DESCRIPTION OF THE INVENTION

The composite storage tank of the invention is particularly useful as anunderground storage tank for the bulk storing of gasoline at retailgasoline service stations. The description which follows is with respectto such a tank. It should be understood the invention is useful forproducing storage tanks of any size and for holding virtually anyliquid, above ground as well as underground.

With reference to FIG. 1, there is shown a male mandrel 10 in theprocess of having a half-section composite storage tank built on itsouter surface. As typical in the industry, a half-section of the storagetank with a domed end cap is formed on the mandrel. The mandrel iscollapsed and the formed half-section tank set aside. Anotherhalf-section tank is formed in the same manner and removed from themandrel. Finally, the two half-section tanks are aligned and permanentlysealed together to form a full tank. A manway is formed on thecylindrical wall as the half-section is being built or subsequentlyadded. Piping access holes may be added.

The mandrel 10 and its mode of operation is conventional. The mandrelhas a cylindrical shape with a diameter approximate that desired for thestorage tank, typically from about four feet to about ten feet. Anelectric motor and drive system (not shown) rotate the mandrel at apredetermined speed.

Generally, in accord with this invention, a viscous resinous corematerial and a set of discrete filament bundles are applied to themandrel 10 and cured to form the composite wall storage tank. Initially,a mold release agent is applied to the mandrel to facilitate removal ofthe half-section tank when formed. Thereafter, an optional thoughpreferred gel coat is applied to the mandrel, followed by application ofviscous resinous core material and filament bundles as described indetail in following paragraphs. The viscous resinous core material andfilament bundles are applied separately to the mandrel as shown or thefilament bundles are run through a trough of the resinous core materialand are simultaneously applied to the mandrel.

Still with reference to FIG. 1, a bundle dispensing means comprises aset of vertically disposed spools 11, a collecting guide 12, and tensionshafts 13. Six spools are depicted. A greater number of spools can beused for increased production time. A lesser number of spools also canbe used, including a single spool, though is less preferred because of aconsequent increase in time needed to form the half-section tank. Thepositions of the tension shafts relative to another are adjusted toincrease or decrease the amount of tension exerted on the bundles. Theamount of tension on the filament bundles during the winding step helpsto determine the degree of their embedding into the viscous resinouscore material. As evident, the spools, collecting guide and tensionshafts are mounted on a cart 14. The cart is motorized and moveslaterally at a predetermined speed to wind the filament bundles in asingle ply around the mandrel as it rotates. One or more plys of thefilament bundles are wound onto the mandrel, preferably at an about 5-30degree angle to the longitudinal axis of the mandrel as it rotates. Aresin spray means 15 moves laterally along the longitudinal axis of themandrel at a predetermined speed as the filament bundles are being woundonto the mandrel's surface.

In accord with this invention, the flow characteristics of the resinousmaterial must be carefully controlled to prevent total resinimpregnation or full wet-out of the filament bundles. That is, resinencapsulation of each of the filament bundles is desired, butsignificant resin penetration into the interiors of the bundles isavoided. The interiors of the filament bundles must remain substantiallyresin-free to provide flow paths for the detection of leakage into orthrough the composite wall as further discussed below.

The filament bundles used in the invention have several continuousfilaments which are preferably discrete filaments loosely held togetheras tows. The individual filaments can also be loosely twisted togetherto form a full bundle. Further, the individual filaments can be looselytwisted together to form several filament yarns which are then formedinto the bundles. Common forms and common nomenclature for the filamentbundles include cords, lines, ropes, yarns, strings and twines.

Individual filaments of most interest because of long lasting strengthare manufactured filaments made from polymeric synthetic resins orglass. Examples of suitable filaments used to form the bundles includeacrylic, nylon, polyester, rayon, acetate, triacetate, olefin and glassfilaments. The filaments are solid or hollow. Filaments of most interestare substantially inelastic. However, elastic filaments such as madefrom an open-cell foam material are feasible. The filaments typicallyhave denier's, i.e. diameters of from about 3.5 microns to about 24microns, through smaller or larger diameter filaments are feasible. Theindividual filaments can as well have another cross-section shape, e.g.rectangular or oblong. The filaments are also made from natural fiberssuch as cotton and wool, though are less preferred because of theirability to absorb moisture and swell over time to the extent leakdetection flow problems within the bundles are created. The preferredmanufactured filaments are more durable. Highly preferred are fiberglassfilaments because of their strength characteristics.

The width and thickness of the individual filament bundles can varywidely. Preferably, they are about 30 mils about 750 mils in width, andabout 20 mils to about 500 mils in thickness. Preferred filament bundlesare formed from fiberglass filaments wherein the individual filamentshave deniers ranging from about 3.5 microns to about 24 microns.

One purpose of the discrete filament bundles is to provide a meanswhereby communication paths are created within the composite wall.Another purpose of the filament bundles is to strengthen the wall. Toaccomplish these objectives, it is important that the density of thefilaments which make up the bundles and the flow characteristics of theliquid viscous resinous core material used in forming the composite wallbe coordinated so that total resin penetration through the individualfilaments is avoided to provide flow paths in the mid-portion of eachbundle.

Examples of resinous core materials used in the invention includepolyesters, vinylesters, polyurethanes and polyepoxides. Other resinsare available and can be used. As aforementioned, the resinousmaterial's flow characteristics prior to application are adjusted to beviscous and substantially non-flowable once a shearing force is removed.This non-flowable characteristic is partially responsible for preventingsignificant resin penetration to the interior of each filament bundle. Ahigh level of thixotropic agent is included with the liquid resinouscore material prior to application. Preferably, for each 100 parts byweight (pbw) resinous material there is from about 1 pbw to about 10 pbwof the thixotropic agent. Examples of suitable thixotropic agentsinclude fumed silica, Thixcin R and ST, and Attagel 50 mineralthickener. Fillers are also preferably included with the resinousmaterial at a level of from about 50 pbw to about 200 pbw per 100 pbw ofthe resinous material to aid in reducing resin flow into the bundles.Examples of suitable fillers are calcium carbonate, silica flour,bentonites, and powdered clays.

FIGS. 2 and 3 show the formation of a section of wall 20 while still onthe mandrel 10. As shown in FIG. 2, initially a thin layer of theviscous resinous core material 21 is applied to the mandrel 10. Whilethe first application of the resinous core material is still in a tackyor gelled state, the discrete filament bundles 22 are wound onto theviscous resinous core material and partially embedded therein.Preferably, the thin layer is first gelled to prevent the filamentbundles from being forced to the mandrel's surface during the windingstep. Tension imparted to the filament bundles by the tension shaftsduring the winding process also can be adjusted to aid somewhat in thisregard. As shown in FIG. 2, the filament bundles have the firstapplication of resinous core material on three sides and partially overa top surface. Thereafter, and with reference to FIG. 3, anotherapplication of viscous resinous material 21 is made. Its flowcharacteristics also are adjusted to result in no or only minimalpenetration. Still with reference to FIG. 3, it is evident that thefilament bundles 22 are encapsulated by the resinous core material 21.However, the interior of each of the bundles is substantiallyresin-free.

Most importantly, the resinous material's lack of significantpenetration into the filament bundles is such as to leave themid-portion of each bundle in a substantially unbonded state. Thisensures the continuous closed communication paths which are needed forleak detection purposes. Some resin penetration into a filament bundleis possible, though each bundle's mid-portion is in the substantiallyunbonded state.

FIG. 4 illustrates a section of wall 25 of a composite storage tankremoved from the mandrel. In its formation, a gel coat 26 is initiallyapplied to the mandrel. Thereafter a viscous resinous core material 27and discrete filament bundles 28 are applied. The gel coat 26 isconventional in nature. It comprises a gasoline-resistant resin and maybe filled with a very fine synthetic fiber. As known in the industry, aresin-rich coat with C-Vail can be used instead of the gel coat. Ineither case, enhanced chemical resistance to a stored liquid isobtained. The gel coat is typically about 10 mils to about 40 milsthick. The balance of the composite wall is built as above describedwith respect to FIGS. 1-3, though the viscous resinous core materialadditionally includes a reinforcing material. Examples of such materialsare a fibrous material such as fiberglass and chopped fiberglassstrands. As well known in the tank building industry, the reinforcingmaterial adds strength and thickness to a FRP (fiberglass reinforcedplastic) wall. It can be mixed in with the viscous resinous corematerial or more preferably separately sprayed along with the viscousresinous core material in the wall's formation. Preferably, twoapplications of the resinous core material and fibrous material are usedas depicted by the dotted lines.

FIG. 5 illustrates a section of wall 30 of a highly preferred compositestorage tank. The section of wall 30 comprises a gel coat 31, an innerfibrous reinforced resinous material 32, a viscous resinous corematerial 33 with discrete filament bundles 34 embedded therein, and anouter fibrous reinforced resinous material 35. The inner and outerlayers of fibrous reinforced resinous material substantially enhance thestrength and dimensional stability of the composite wall and their useis preferred for this reason. The gel coat, or alternatively resin-richcoat, and viscous resinous core material with the filament bundles areapplied to a mandrel as above described with respect to FIG. 4.

The fibrous reinforced resinous materials 32 and 35 are formed fromconventional fibrous material and resinous material in a manner wellknown in the FRP industry. The above described resins used in theresinous core material are used here as well. Preferably, the fibrousmaterial in the form of chopped strands and the resinous material areseparately sprayed simultaneously onto the viscous resinous corematerial 33 while in an uncured tacky or fully cured state. Optionally,though less preferred, either layer of fibrous reinforced resinousmaterial can be omitted without losing the benefit of the remaininglayer.

FIG. 6 illustrates a section of wall 40 of another composite storagetank wall. The wall section comprises a gel coat 41, a resinous corematerial 42 with discrete filament bundles 43 embedded therein andadditional resinous core material 44 with discrete filament bundles 45embedded therein. The raw materials and modes of application are thesame as above described. As evident, there can be some overlapping ofthe filament bundles at random spots throughout the tank wall. Asfurther discussed, intercommunication of individual flow paths withinthe filament bundles occurs with the overlapping bundles and actuallycan enhance speed of leak detection.

FIGS. 7 and 8 show a section of wall 50 of an alternative compositestorage tank wall. Two sets of filament bundles are used, though atsubstantially right angles to one another. The wall section is feasible,though is not preferred because of increased labor and equipment costsin applying the filament bundles along the mandrel's longitudinal axis.The wall comprises a gel coat 51, resinous core material 52 withdiscrete filament bundles 53, and additional resinous core material 54with filament bundles 55. A resin-rich coat can be used in place of thegel coat. The resinous core materials are viscous when applied asdescribed above to ensure that the interiors of the filament bundlesremain substantially resin-free. The viscous resinous material can besequentially applied to ensure encapsulation of the filament bundles.While not shown, some overlapping of the respective filament bundlesoccurs so that individual flow paths are interconnected. This canenhance the ability to detect leakage.

FIG. 9 illustrates a section of wall 60 of still another compositestorage tank wherein a foamed layer of resinous core materialencapsulates filament bundles. The wall includes a gel coat 61, afibrous reinforced resinous material 62, a foamed resinous core material63 with filament bundles 64, and additional fibrous reinforced resinousmaterial 65. The materials and modes of application of the gel coat andthe two fibrous reinforced resinous materials are similar to thatdescribed above with reference to FIGS. 6-8. However, the resinous corematerial includes a foaming agent such that after its application withthe filament bundles a closed cell foam encapsulating the filamentbundles results. The advantage of the described wall is a consequentwall stiffness and thickness.

FIGS. 10 and 11 show a composite storage tank 70 of the invention whichis particularly preferred. The cylindrical-shaped main body 71 of thetank is constructed as above described with reference to FIG. 4. Domedend caps 72 are secured to each end of the main body 71. A set ofsupport ribs 73 is positioned to extend circumferentially around thetank wall. They are conventional in nature and are used to enhance thestrength of the tank against both internal and external forces. A manway(not shown) and/or piping access holes (not shown) are added for theirknown functions.

It should be evident that mid-portions of the filament bundles of thecomposite wall serve as closed communication paths. Such paths provide ameans whereby any leakage of liquid or air can be conveyed and/ordetected. Damage of the storage tank system's composite wall will causeleakage which will lead to one or more of the communication paths. Thecommunication paths in turn all lead directly or indirectly to amonitoring station. High speed communication paths are preferably addedin selected areas of the composite wall to intersect with theaforedescribed filament bundle paths. Preferably, the high speedcommunication paths are found in the storage tank's cylindrical wall andend caps. They further extend longitudinally along the top and/or bottomof the tank to intersect with the flow paths within the filamentbundles. These high speed communication paths are provided by, forexample, apertured tubes, a fibrous strip material, or a foam-type stripmaterial. The strip materials are capable of readily transmitting liquidor air. One such high speed communication path 74 made of a foam-typestrip material is seen best in FIG. 11. It is necessary that the highspeed communication paths be in fluid communication with the interiorsof the encapsulated filament bundles to achieve a quick detection ofleakage in any part of the storage tank. Accordingly, a part of anyencapsulated filament bundle where a cross-over or cross-under of a highspeed communication path occurs is bared to access its interior. Simplyremoving a part of the encapsulating resin where the cross-over orcross-under occurs is feasible. Alternatively, the filament bundlecross-over or cross-under areas are not coated with the viscous resinouscore material, e.g. by masking over such areas.

Various means of detecting leakage in the composite storage tank of theinvention are used. Reference is made to my U.S. Pat. No. 4,869,386,Column 5, line 36 to Column 6, line 46 for examples of monitoring means,the disclosure of which is hereby incorporated by reference.

The invention has been described with particular reference to thedrawings. A detailed description has been given for production of thecomposite wall. The composite wall is built on a mandrel as shown toform a storage tank. The composite wall can also be built on othersupport surfaces, such as inside a female mold or on the inside oroutside of an existing tank as in a retrofit situation. Other manners ofapplying the filament bundles and resinous materials can be used. Forexample, the individual filament bundles can run vertically,horizontally, and at angles to the axis of the storage tank. These alsocan be multiple layers of bundles. The resinous material whichencapsulates the filament bundles to form the flow paths can be appliedall in one step or several steps and by different techniques. Forexample, the resinous core material can be sprayed or poured onto thefilament bundles to cover primarily the sides and tops of the bundles,while the bottom of the filament bundles are covered by a previouslyapplied gel coat or fibrous reinforced resinous material. Thus, totalencapsulation is achieved by the combined gel coat or fibrous reinforcedresinous material and viscous resinous core material. These and othervariations of the invention of an obvious nature are considered withinthe scope of the appended claims.

I claim:
 1. A composite storage tank having double wall characteristics, comprising:(a) a cylindrical-shaped tank wall of a resinous core material and discrete filament bundles extending continuously within the tank wall, wherein the resinous core material encapsulates the filament bundles, yet does not significantly penetrate into the filament bundles so that interiors of the filament bundles are substantially resin-free; and (b) end caps at each end of the cylindrical-shaped tank wall to form the storage tank.
 2. The composite storage tank of claim 1 further comprising a gel coat or resin-rich coat on an interior of the wall to provide stored chemical resistance.
 3. The composite storage tank of claim 2 further wherein at least one fibrous reinforced resinous material is formed adjacent the resinous core material.
 4. The composite storage tank of claim 1 further comprising an additional resinous core material with filament bundles embedded therewithin over the first resinous core material with filament bundles embedded therein.
 5. The composite storage tank of claim 1 wherein the resinous core material is foamed.
 6. The composite storage tank of claim 1 further comprising a set of spaced support ribs extending circumferentially around the cylindrical-shaped tank wall.
 7. The composite storage tank of claim 1 further comprising a high speed communication path extending longitudinally along the cylindrical-shaped wall and in fluid communication with the interiors of the filament bundles. 